In my U.S. Pat. No. 4,055,019 there is described and claimed a planar element for use as part of a constructional toy comprising a plurality of such elements. The planar element has at least one side margin comprising first and second spaced projections which define therebetween a recess, that side face of the recess defined by an inner side face of said first projection being an abutment face, one of the side face: of the recess opposite the abutment face and the outer side face of the first projection being provided with a hinge socket and the other said face with a corresponding hinge pip, and a slot in said first or second projection extending inwardly from the outer edge of the projection to form a resilient spring at one side of said recess, the arrangement being such that when two such elements are connected together by inserting the first projection of each element in the recess of the other element said springs will provide sufficient resilience to enable the hinge pip of each element to be sprung into the hinge socket of the other element to thereby hingedly connect the elements together.
Constructional toys comprising a plurality of planar elements according to my said Patent are sold under the Registered Trademark "Polydron" and have been commercially successful, particularly in the field of education. However, it has become apparent that there is a need for a version of such a constructional toy wherein the planar elements comprise simply an outer peripheral frame with no material in the centre thereof so that when a plurality of such elements are hingedly connected together edge-to-edge to form a polyhedral structure not only the outside but also the inside of the polyhedral structure can be examined. Whilst planar elements comprising simply an outer peripheral frame were contemplated in my aforesaid Patent, if it is desired to provide such an element wherein the outer peripheral frame is relatively narrow, and therefor aesthetically more pleasing, then the provision of a said inwardly extending slot to provide said resilient spring in each if the margins of the planar element becomes impractical because it so weakens the element that it is liable to break in use.